The present invention relates to a power supply device for driving liquid crystal, together with a liquid crystal device and electronic equipment which use that power supply device.
Conventional methods of reducing the required current for a power supply device which is used for driving liquid crystal have been disclosed in Japanese Patent Applications Laid-open No. 6-324640, No. 7-98577, No. 9-43568, and the like. An example of a conventional power supply device for driving liquid crystal is shown FIG. 7.
A power supply device for driving liquid crystal 701 shown in FIG. 7 has a voltage division circuit 702, two first impedance conversion circuits 703, and two second impedance conversion circuits 704.
The voltage division circuit 702 contains resistors 706 to 710 and generates voltages V1 to V4 by dividing a voltage between a source voltage VDD and a reference voltage for driving liquid crystal VLCD.
When the source voltage VDD is a voltage V0 and the reference voltage for driving liquid crystal VLCD is a voltage V5, voltages V0 to V5 correspond to voltage levels in the driving waveform for scan electrodes (or common electrodes) COM0, COM1, and COMX shown in FIG. 13 and also for signal electrodes (or segment electrodes) SEG1 to SEG4 shown in FIG. 14.
The first impedance conversion circuit 703 is formed by voltage follower connection of an operational amplifier consisting of a constant current circuit 801, P-type differential amplification circuit 802, and output circuit 803 as shown in FIG. 8. An N-type transistor 805 in the output circuit 803 forms a current source by receiving a constant bias voltage from the constant current circuit 801, thereby providing a load for the P-type transistor 804.
The characteristics of the first impedance conversion circuits 703 which generate the voltages V1 and V3 are determined by taking into account the direction of movement of electric charges in the scan electrodes (or common electrodes) or the signal electrodes (or segment electrodes) to which the voltage V1 or V2 is applied. Specifically, as indicated by 1102 in FIGS. 13 and 14, positive charges to be moved from the first impedance conversion circuits 703 to the electrodes is larger in amount than negative charges. For this reason, a P-type transistor 804 which causes a current to flow into the electrodes is used as an active element in the first impedance conversion circuits 703.
The second impedance conversion circuit 704 is formed by voltage follower connection of an operational amplifier consisting of a constant current circuit 901, N-type differential amplification circuit 902, and output circuit 903 as shown in FIG. 9. A P-type transistor 904 in the output circuit 903 forms a current source by receiving a constant bias voltage from the constant current circuit 901, thereby providing a load for the N-type transistor 905.
The characteristics of the second impedance conversion circuits 704 which generate the voltages V2 and V4 are also determined by taking into account the direction of movement of electric charges in the scan electrodes (or common electrodes) or the signal electrodes (or segment electrodes) to which the voltage V2 or V4 is supplied. Specifically, as indicated by 1201 in FIGS. 13 and 14, negative charges to be moved from the second impedance conversion circuits 704 to the electrodes is larger in amount than positive charges. For this reason, an N-type transistor 905 which causes a current to be drawn from the electrodes is used as an active element in the second impedance conversion circuits 704.
Among the divided voltages V1 to V4 in the voltage division circuit 702, the voltages V1 and V3 are repectively input to the plus terminals of the first impedance conversion circuits 703, and the voltages V2 and V4 are respectively input to the plus terminals of the second impedance conversion circuits 704. The impedance conversion of the voltages V1 to V4 can be carried out in this manner, thereby generating voltages for driving liquid crystal V1 to V4.
Conventional power supply devices for driving liquid crystal use an active load for the output circuit of an impedance conversion circuit to reduce current flowing through loading transistors, thereby reducing required current flowing through the impedance conversion circuit.
For maintaining display quality while limiting the amount of current flowing in the loading transistors through the impedance conversion circuits, the above-described load current must be supplemented. For this reason, it has been required to provide a capacitor element 705 between the output line for each of the voltages V1 to V4 and the output line for the voltage V0 (VDD), as shown in FIG. 7. The above load current can be supplemented by discharging the charges from the capacitor element 705.
However, the capacitor element 705 has to be provided outside the power supply device for driving liquid crystal, because the capacitor element 705 has a large volume.
Downsizing and cost reduction are strongly demanded factors for electronic equipment, particularly for portable electronic equipment having a built-in liquid crystal device, so that the display quality is required to be maintained while reducing the number of parts such as capacitor elements.
The present invention has been devised to solve the above problems and has as an objective therof the provision of a power supply device for driving liquid crystal which enables low current comsumption, together with a liquid crystal device and electronic equipment using such a power supply device.
Another objective of the present invention is to provide a power supply device for driving liquid crystal which enables to omit parts such as a capacitor element while maintaining display quality, together with a liquid crystal device and electronic equipment using such a power supply device.
The power supply device for driving liquid crystal of the present invention which generates N numbers of liquid crystal drive voltages between first and second reference voltages, comprises: a voltage division circuit which divides a voltage between the first and second reference voltages to generate N pairs of first and second voltages comprising N numbers of first voltages each of which is equal to or higher than each of the N numbers of liquid crystal drive voltages, and N numbers of second voltages each of which is equal to or lower than each of N numbers of liquid crystal drive voltages, when the first voltage is not equal to the second voltage in each pair; and N numbers of impedance conversion circuits which generate N numbers of impedance transformed liquid crystal drive voltages based on the N pairs of the first and second voltages.
Each of the N numbers of impedance conversion circuits comprises: a voltage follower type of differential amplification circuit to which a pair of the first and second voltages among the N pairs of the first and second voltages is input; and an output circuit including a P-type transistor and N-type transistor connected in series between a first power supply line for the first reference voltage and a second power supply line for the second reference voltage, and having an output terminal which is connected between the P-type transistor and N-type transistor and outputs one of the N numbers of liquid crystal drive voltages.
On-and-off operation of the N-type transistor is controlled by the first output voltage from the differential amplification circuit, and on-and-off operation of the P-type transistor is controlled by the second output voltage from the differential amplification circuit.
In each impedance conversion circuit according to the present invention, a first and a second output voltage, each differing from the other, are output from a voltage follower type of differential amplification circuit to which a first and a second voltage, each differing from the other, are input. In each impedance conversion circuit, the voltage for driving liquid crystal is generated by independently controlling the on-and-off operation of the P-type and N-type transistors of the output circuit by the first and second output voltages.
The differential amplification circuit may turn on the N-type transistor when an output voltage of the output terminal is higher than the first voltage, turn on the P-type transistor when an output voltage of the output terminal is lower than the second voltage, and turn off both the P-type and N-type transistors when an output voltage of the output terminal is between the first and second voltages. This operational mode prevents both the P-type and N-type transistors from being turned on at the same time, thereby preventing a short circuit current from flowing via the P-type and N-type transistors and reducing current consumption.
Current drive capabilities of the P-type and N-type transistors of the output circuit may be substantially equivalent. This enables the voltage to promptly converge to the liquid crystal drive voltage irrespective of polarity (positive or negative) of the charge to be transferred from the electrodes of a liquid crystal panel to be driven to the impedance conversion circuit. In addition, a sufficient quantity of load current can be secured even if no capacitor elements are connected. Furthermore, when an overload in the reverse direction is applied by surge or the like, a required amount of charge can be immediately supplied by the N-type or P-type transistor, whereby anti-noise properties can be improved, resulting in high display performance.
A potential difference between voltages of a pair of the first and second voltages may be variable in the voltage division circuit. Variation in the characteristics of the differential amplifier, particularly variation in the offset voltage between the input and output voltages can be controlled in this manner.
A potential difference between voltages of a pair of the first and second voltages may be larger than the absolute value of an offset voltage between input and output voltages of the differential amplification circuit. Otherwise a potential difference might not be created between the first and second voltages, even if the first and second voltages are different.
The differential amplification circuit may comprise: an N-type voltage follower differential amplification circuit which receives the first voltage and applies the first output voltage to a gate of the N-type transistor; and a P-type voltage follower differential amplification circuit which receives the second voltage and applies the second output voltage to a gate of the P-type transistor.
In this case, a potential difference between voltages of a pair of the first and second voltages may be larger than the sum of the absolute value of a first offset voltage between input and output voltages of the N-type differential amplification circuit and the absolute value of a second offset voltage between input and output voltages of the P-type differential amplification circuit. The potential difference between the first and second output voltages can be ensured in this manner.
At least one of the N impedance conversion circuits may be connected between the output terminal and the second power supply line in parallel with the N-type transistor, and may further comprise an N-type transistor for a constant current having a gate to which a constant bias voltage is applied.
This configuration is effective when negative charges to be transferred from the electrodes for driving liquid crystal to the impedance conversion circuit is larger than positive charges to be transferred in a similar way. It is because negative charges can be drawn by driving the N-type transistor for a constant current.
At least another one of the N impedance conversion circuits may be connected between the first power supply line and the output terminal in parallel with the P-type transistor, and may further comprise another P-type transistor for a constant current having a gate to which a constant bias voltage is applied.
This configuration brings about a greater advantage when positive charges to be transferred from the electrodes for driving liquid crystal to the impedance conversion circuit is larger than negative charges to be transferred in a similar way. It is because positive charges can be drawn by driving the P-type transistor for a constant current.
At least one of the N numbers of impedance conversion circuits may have the first voltage among a pair of the first and second voltages set substantially equivalent to one of the N numbers of liquid crystal drive voltages.
In this manner, the voltage can converge relatively promptly to the inherent voltage for driving liquid crystal, even if a voltage lower than the liquid crystal drive voltage is applied to the output terminal of the impedance conversion circuit while the liquid crystals are driven.
At least another one of the N numbers of impedance conversion circuits may have the second voltage among a pair of the first and second voltages set substantially equivalent to another one of the N numbers of liquid crystal drive voltages.
In this manner, the voltage can converge relatively promptly to the inherent voltage for driving liquid crystal, even if a voltage higher than the liquid crystal drive voltage is applied to the output terminal of the impedance conversion circuit while the liquid crystals are driven.
A liquid crystal device of the present invention comprises: the above-described power supply circuit for driving liquid crystal; a liquid crystal panel in which scanning electrodes and signal electrodes are formed; a scanning electrode drive circuit which drives the scanning electrodes based on power supply from the power supply circuit for driving liquid crystal; and a signal electrode drive circuit which drives the signal electrodes based on the power supply from the power supply circuit for driving liquid crystal.
Electronic equipment of the present invention comprises the above-mentioned liquid crystal device.
The liquid crystal device and electronic equipment of the present invention are particularly useful for a portable electronic instrument having a liquid crystal device, because of a low current consumption due to prevention of short circuit current from flowing and miniaturization due to elimination of installed parts such as a capacitor element.